1. Field of the Invention
The present invention relates to a laser system in which a laser beam generated through a laser medium is emitted from a stable resonator.
2. Description of the Prior Art
FIG. 1 is a view showing the construction of a conventional laser system which is disclosed, for example, in Japanese Patent Laid Open No. 152777/89. In the same figure, the reference numeral 1 denotes a concave total reflector formed of Cu for example; the numeral 2 denotes a coupling mirror formed of ZnSe for example and disposed in an opposed relation to the total reflector 1; numeral 11a denotes a partial reflection film formed on the opposed surface to the total reflector 1 of the coupling mirror 2; numeral 11b denotes a partial reflection film provided around the partial reflection film 11a and having a partial reflectivity lower than that of the partial reflection film 11a; and numeral 12 denotes an antireflecting film formed on the surface of the coupling mirror 2 on the side opposite to the side where the partial reflection films 11a and 11b are provided. Numeral 3 denotes an aperture formed in front of the total reflector 1 and also in front of the coupling mirror 2 within a stable resonator. Numeral 4 denotes a laser medium. In the case of a gas laser such as CO.sub.2 laser, the laser medium is a gas medium pumped by electric discharge for example, or in the case of a solid-state laser such as YAG laser, the laser medium is a solid-state medium pumped by a flash lamp for example. Numeral 5 denotes a housing; numeral 6 denotes a laser beam generated in the interior of a stable resonator composed of the mirrors 1 and 2; and numeral 7 denotes a laser beam taken out to the exterior of the laser oscillator by the coupling mirror 2.
Description is now directed to the operation of such conventional laser system. The mirrors 1 and 2 constitute a stable resonator, in which the laser beam 6 is amplified by the laser medium 4 while propagating between the mirror, 1 and 2. At the same time, part of the laser beam 6 passes through the partial reflection films 11a, 11b and further through the antireflecting film 12 which are provided in the coupling mirror 2, and is extracted as laser beam 7 to the exterior of the laser oscillator. If the extracted laser beam 7 thus is converged using a lens or the like, there will be obtained a converged beam of a crown shape, namely a laser beam having a remarkably high intensity distribution at the central part, which can be used for cutting or welding iron plates, etc.
Not only the output of laser beam but also the mode thereof is an important factor. A suitable mode is selected according to the total reflector 1 and coupling mirror 2 which constitute a resonator, as well as the resonator length and both the aperture diameter in the resonator and the resonator length. In the case of a CO.sub.2 laser processing machine, there usually is employed TEM.sub.00 (Transverse Electro-Magnetic) mode or TEM.sub.01 * mode for the cutting of an iron plate or the like, in which TEM represents a light wave from an electromagnetic field perpendicular to the propagating direction, and .sub.00 or .sub.01 represents the number of transverse modes.
In the stable resonator of the conventional laser system shown in FIG. 1 and constructed as above, the aperture 3 is merely for defining an outer diameter of the laser beam 6 and does not perform the mode selection. The partial reflectors 11a and 11b also merely function to pass a portion of the laser beam 6 therethrough and cause the laser beam to assume an inner-stuffed state (namely, a laser beam having a uniform intensity distribution). In such a construction, therefore, the mode selection is not performed strictly and a laser beam is generated in a multi-mode wherein the beam quality is poor.
In the case of the laser system shown in FIG. 2, which is generally used, apertures 3 provided within a stable resonator perform the mode selection and can generate TEM.sub.00 mode wherein the beam quality is good, but at the same time the pumping space is limited, so there has been the problem that the laser output is limited.
The laser output can be increased by increasing the applied electric power, or laser gain. However, since the diameter of the apertures 3 is fixed, the increase of laser gain leads to an increase in intensity of the laser beam incident on a partial reflector 11 of a coupling mirror 2. On the other hand, an upper limit of output at the time of extracting the laser beam from the coupling mirror 2 is determined by the light-resisting strength of the coupling mirror 2. For example, in the case where laser beam is generated in TEM.sub.00 mode, a maximum output of a laser beam 7 capable of being extracted stably is 2 kW. Generating a laser beam of greater than 2 kW in TEM.sub.00 mode and extracting it stably are in many cases difficult due to the problem related to the light-resisting strength of the coupling mirror. Even in the laser processing machines presently available on the market, a maximum laser beam output is 4 kW, and various improvements are applied to the coupling mirror for attaining this maximum output. This is the actual situation.